Among oxide superconductive materials developed so far, it is characterized that an oxide superconductive material of Y (yttrium) system has a high pinning force, and that of Bi (bismuth) system has a property of easily satisfying a transport critical current density (defined "Jc" hereinafter) even in a high magnetic field at a temperature of 4.2 K. These oxide superconductive materials have been studied to be applied to wires, bulks, films, etc.
However, the Y system superconductive material has a disadvantage in that the temperature margin is narrow on the basis that it is used in the vicinity of 77 K., because a critical temperature thereof is in the vicinity of 93 K. It has further disadvantages in that Jc inside particles thereof is low at a high temperature, and the bonding property on the grain boundary is poor not to meet the requirement of a transport Jc.
On the other hand, 1212 phase, 1223 phase and 1234 phase are known as crystal structures including 2, 3 and 4 layers of Cu--O for a Tl system superconductive material of Tl--O single layer, the basic composition of which is Tl--Sr--Ca--Cu--O. It is known in this superconductive material that, when Tl site of the basic composition is replaced by Pb, a critical temperature is increased, and, when Sr (strontium) is replaced by Ba (barium), mono-phase is easily obtained. The highest temperature state is at 100 K. for 1212 phase, and is at 122 K. for 1223 and 1234 phases.
It is also known that 1223 phase of an oxide superconductive material of Tl and Pb system, that is, (Tl.sub.x Pb.sub.1-x).sub.1, (Sr.sub.y Ba.sub.1-y).sub.2 Ca.sub.2 Cu.sub.3 O.sub.2, and 1234 phase thereof, that is (Tl.sub.x Pb.sub.1-x).sub.1, (Sr.sub.y Ba.sub.1-y).sub.2 Ca.sub.3 Cu.sub.4 O.sub.2 have a large pinning force in a high magnetic field at a temperature of 77 K. and an improved transport Jc (0&lt;x.ltoreq.1 and 0.ltoreq.y.ltoreq.1).
In manufacturing oxide superconductive wires, bulks, films, etc. having the Tl and Pb single layer 1223 and/or 1234 phases as a main component, oxides or carbonates are mixed by a predetermined mole ratio, and is fired by a heating condition, according to which 1223 phase or 1234 phase becomes a main phase. Then, the fired product is pulverized to provide precursor powders (provisionally fired powders).
In manufacturing a metal-sheathed oxide superconductive wire, for instance, the precursor powders having 1223 phase and/or 1234 phase as a main component are supplied into a metal pipe which is then processed repeatedly by drawing and heating to provide a predetermined wire of Tl and Pb system.
In manufacturing a superconductive film, for another instance, the precursor powders having 1223 phase and/or 1234 phase as a main component are mixed with binder, dispersion material, diluent, etc. to provide pasted substance which is then provided in film on a metal or ceramic substrate by the doctor blade method, etc, and the substrate thus coated by the pasted substance is processed by heating.
In manufacturing a superconductive material in the form of bulk, on the other hand, the precursor powders having 1223 phase and/or 1234 phase as a main component are shaped to be a predetermined bulk article by compression, and the bulk article is heated by a predetermined temperature. This process may be repeated by predetermined times.
According to the conventional superconductive material, a starting material of which is the precursor powders having 1223 phase and/or 1234 phase as a main component to be processed by thermal and/or compression treatments, however, there are disadvantages in that finally synthesized 1223 phase and/or 1234 phase of Tl and Pb system are small granular grains, because crystal particles thereof are less grown, and in that no crystal phase is substantially oriented, and different phases are considerably found therein. For these reasons, a bonding property on the grain boundaries is poor, so that an advantage in which 1223 phase and/or 1234 phase have a large pinning force essentially is not utilized. Consequently, where a magnetic field is applied to the superconductive material at a temperature of 77 K., the transport Jc is slightly deteriorated.
Accordingly, it is an object of the invention to provide an oxide superconductive material of Tl--O system and a method for manufacturing the same in which the transport Jc property is enhanced even in the vicinity of a temperature of 77 K.
According to a feature of the invention, an oxide superconductive material of Tl and Pb system, comprises;
superconductive phase and non-superconductive phase, said non-superconductive phase being less in volume proportion than 30%, said superconductive phase being of superconductive crystals comprising Tl, Pb, Sr, Ba, Ca, and Cu and having a structure of 1223 phase and/or 1234 phase, and a long axis length of said crystal being more than 30 .mu.m.
According to another feature of the invention, a method for manufacturing an oxide superconductive material of Tl and Pb system, comprises the steps of:
preparing a powdery precursor having a crystal structure of 1212 phase as a main component, said powdery precursor comprising Tl, Pb, Sr, Ba, Ca and Cu; and
processing said powdery precursor mechanically and thermally to provide said oxide superconductive material having a crystal structure of 1223 phase and/or 1234 phase as a main component.